•Synthesis of a new readily-reduced mixed iron/cobalt perovskite.•Detailed study of physical properties of new material.•Identification of possible new membrane material.BaFe0.9-xY0.1CoxO3-δ (0 ≤ x ≤ 0.15) is shown to adopt the cubic perovskite structure at temperatures up to 1400 K in air. The oxygen vacancy concentration increases with both cobalt content and temperature, with BaFe0.75Y0.1Co0.15O2.35 being the most oxygen-deficient composition observed. At 700 K the conductivity is ∼2 S cm−1 across the whole composition range. Measurements of the Seebeck coefficient show that holes are the dominant carrier. The coefficient of thermal expansion is essentially constant above 800 K, but not at lower temperatures. Chemical reactivity tests, along with the coefficient of expansion, show that these perovskites would not be suitable electrodes for fuel cells with fluorite electrolytes but they might serve as permeable membranes in catalytic reactors.

Polycrystalline samples of Ln2CoGe4O12 (Ln = Gd, Tb, Dy, Ho or Er) and LnBCoGe4O12 (B = Sc or Lu) have been prepared and characterised by a combination of magnetometry, 155Gd Mössbauer spectroscopy and, in the case of Tb2CoGe4O12 and TbScCoGe4O12, neutron diffraction. The holmium- and erbium-containing compositions remain paramagnetic down to 2 K, those containing dysprosium behave as spin glasses and the terbium and gadolinium-containing compounds show long-range magnetic order with transition temperatures below 4 K in all cases. The data can be rationalized qualitatively in terms of the interplay between magnetic anisotropy and crystal field effects.

•Rapid spin relaxation occurs over a wide temperature range below the Néel temperature.•Magnetic moments on two cation sublattices develop at significantly different rates.•Centrosymmetric structure is confirmed by electron microscopy.A polycrystalline sample of Ba3Fe2TeO9 having the 6H perovskite structure has been prepared in a solid-state reaction and studied by a combination of electron microscopy, Mössbauer spectroscopy, magnetometry, X-ray diffraction and neutron diffraction. Partial ordering of Fe3+ and Te6+ cations occurs over the six-coordinate sites; the corner-sharing octahedra are predominantly occupied by the former and the face-sharing octahedra by a 1:1 mixture of the two. On cooling through the temperature range 18 < T/K < 295 an increasing number of spins join an antiferromagnetic backbone running through the structure while the remainder show complex relaxation effects. At 3 K an antiferromagnetic phase and a spin glass coexist.On cooling an increasing number of spins join an antiferromagnetic backbone running through the structure while the remainder show complex relaxation effects.Download high-res image (122KB)Download full-size image

Polycrystalline samples of CaLn2Ni2WO9 (Ln=La, Pr, Nd) have been synthesized and characterised by a combination of X-ray and neutron diffraction, electron microscopy and magnetometry. Each composition adopts a perovskite-like structure with a~5.50, b~5.56, c~7.78 Å, β~90.1° in space group P21/n. Of the two crystallographically distinct six-coordinate sites, one is occupied entirely (Ln=Pr) or predominantly (Ln=La, Nd) by Ni2+ and the other by Ni2+ and W6+ in a ratio of approximately 1:2. None of the compounds shows long-range magnetic order at 5 K. The magnetometry data show that the magnetic moments of the Ni2+ cations form a spin glass below ~30 K in each case. The Pr3+ moments in CaPr2Ni2WO9 also freeze but the Nd3+ moments in CaNd2Ni2WO9 do not. This behaviour is contrasted with that observed in other (A,A')B2B'O9 perovskites.Absence of long-range magnetic order in CaLn2Ni2WO6 despite partial ordering of Ni2+ and W6+ cations.Download high-res image (165KB)Download full-size image

Polycrystalline samples in the solid solution ZrMn2−xCoxGe4O12 (x = 0.0, 0.5, 1.0, 1.5 and 2.0) have been prepared using the ceramic method and characterised by a combination of magnetometry, X-ray diffraction and neutron diffraction. They all adopt the space group P4/nbm with a ∼ 9.60, c ∼ 4.82 Å and show long-range magnetic order with transition temperatures, TC, in the range 2 ≤ TC/K ≤ 10. The underlying magnetic structure is the same in each case but the ordered spins lie along [001] when x = 0.0 and in the (001) plane for all other compositions. In all cases the magnetically-ordered phase is a weak ferromagnet although the magnitude of the spontaneous magnetisation and the strength of the coercive field are composition-dependent. The magnetic structure can be rationalized by considering the strengths of the interactions along the distinct M–O–Ge–O–M superexchange pathways in the crystal and the observed magnetic structure is entirely consistent with the predictions of ab initio calculations.

•A-type antiferromagnetic ordering in mixed-metal germanates.•Unusually large isomer shift for Fe3+ in Mössbauer spectrum of YMnFeGe4O12.•Identification of factors that determine the magnetic structure adopted by compounds having this structure type.Polycrystalline samples of LnMnFeGe4O12 (Ln = Y, Eu, Lu) have been prepared using the ceramic method and characterised by a combination of magnetometry, Mössbauer spectroscopy, X-ray diffraction and neutron diffraction. They all adopt the space group P4/nbm with a ~ 9.670, c ~ 4.81 Å and show long-range antiferromagnetic order with transition temperatures 15 ≤ TN/K ≤ 17. The magnetic structure is the same in each case and consists of an A-type ordering of (001) planes; the ordered spins lie in the (001) plane. Comparison with isostructural compounds leads to the conclusion that subtle structural changes play a greater role than the electronic configuration of the cation in determining the magnetic structure.LnMnFeGe4O12 (Ln = Y, Eu, Lu) order as A-type antiferromagnets with 15 ≤ TN/K ≤ 17Download high-res image (104KB)Download full-size image

Polycrystalline samples, prepared by a solid-state route, of compositions in the solid solution CeMn2–xCoxGe4O12 (x = 0.0, 0.5, 1.0, 1.5, and 2.0) were characterized by X-ray diffraction, neutron diffraction, and magnetometry. They adopt space group P4/nbm with a ≈ 9.78 and c ≈ 4.85 Å and become anti-ferromagnetic (x = 0.0, 1.5, 2.0) or weakly ferromagnetic (x = 0.5, 1.0) at 4.2 ≤ T ≤ 7.6 K. The ordered moments lie along [001] when x = 0.0 and in the (001) plane otherwise. The unit cell doubles along [001] when x = 1.5 and 2.0 order anti-ferromagnetically, but the doubling is lost when a first-order metamagnetic transition to weak ferromagnetism occurs on the application of a 10 kOe magnetic field. The ordered moments at 1.6 K for x = 0.0 and 2.0 are 4.61(2) and 2.58(2) μB, respectively; the corresponding effective moments in the paramagnetic phase are 5.91 and 5.36 μB.

Non-equilibrium molecular dynamics has been used to model cation diffusion in stoichiometric Li3N over the temperature range 50 < T/K < 800. The resulting diffusion coefficients are in excellent agreement with the available experimental data. We present a detailed atomistic account of the diffusion process. Contrary to the conclusions drawn from previous studies, our calculations show that it is unnecessary to invoke the presence of a small concentration of intrinsic defects in order to initiate diffusion. The structure can be considered to consist of alternating layers of composition Li2N and Li. As the temperature increases an increasing number of cations leave the Li2N layers and migrate either to the interlayer space or to the Li layer. Those that move into the interlayer space form Li2 dimers with cations in the Li2N layers and those that move into the neighboring layer form dimers with cations therein. The two types of dimer are aligned parallel and perpendicular to [001], respectively and have lifetimes of ∼3 ps. The vacancies so created facilitate rapid diffusion in the Li2N layers and the interlayer cation motion results in slower diffusion perpendicular to the layers.

•Coexistence of spin-glass and ordered phases.•Local inhomogeneity despite long-range inhomogeneity.•Contrasting magnetic properties of SrLa2Ni2TeO9 and La3Ni2SbO9.A polycrystalline sample of SrLa2Ni2TeO9 has been synthesized using a standard ceramic method and characterized by neutron diffraction, magnetometry and electron microscopy.The compound adopts a monoclinic, perovskite-like structure with space group P21/n and unit cell parameters a=5.6008(1), b=5.5872(1), c=7.9018(2) Å, β=90.021(6)° at room temperature. The two crystallographically-distinct B sites are occupied by Ni2+ and Te6+ in ratios of 83:17 and 50:50.Both ac and dc magnetometry suggest that the compound is a spin glass below 35 K but the neutron diffraction data show that some regions of the sample are antiferromagnetic. Electron microscopy revealed twinning on a nanoscale and local variations in composition. These defects are thought to be responsible for the presence of two distinct types of antiferromagnetic ordering.Figure optionsDownload full-size imageDownload high-quality image (193 K)Download as PowerPoint slideThe magnetic properties of SrLa2Ni2TeO9 are discussed in terms of cation ordering in the microstructure.

•2:1 Cation ordering in a trigonal perovskite.•Magnetically ordered trigonal perovskite.•Intergrowth of nanodomains in perovskite microstructure.A polycrystalline sample of perovskite-like Sr3Fe2TeO9 has been prepared in a solid-state reaction and studied by a combination of electron microscopy, Mössbauer spectroscopy, magnetometry, X-ray diffraction and neutron diffraction. The majority of the reaction product is shown to be a trigonal phase with a 2:1 ordered arrangement of Fe3+ and Te6+ cations. However, the sample is prone to nano-twinning and tetragonal domains with a different pattern of cation ordering exist within many crystallites. Antiferromagnetic ordering exists in the trigonal phase at 300 K and Sr3Fe2TeO9 is thus the first example of a perovskite with 2:1 trigonal cation ordering to show long-range magnetic order. At 300 K the antiferromagnetic phase coexists with two paramagnetic phases which show spin-glass behaviour below ~80 K.Sr3Fe2TeO9 has a 2:1 ordered arrangement of Fe3+ and Te6+ cations over the octahedral sites of a perovskite structure and is antiferromagnetic at room temperature.Figure optionsDownload full-size imageDownload as PowerPoint slide

The structure proposed for Li11Nd18Fe4O39−δ (Chen et al. Inorg. Chem. 2012, 51, 8073) on the basis of diffraction and Mössbauer spectral data is compared to that determined previously for Nd18Li8Fe5O39 (Dutton et al. Inorg. Chem.200847, 11212) using the same techniques. The Mössbauer spectrum reported by Chen et al. has been reinterpreted. The newly refined spectral parameters differ significantly from the published values but are similar to those reported for Nd18Li8Fe5O39. The relative areas of the three components indicate that iron cations occupy the 2a, 8e, and 16i sites in space group Pm3̅n, in disagreement with the model determined from neutron diffraction by Chen et al. in which only the 2a and 8e sites are so occupied. The relationship between Li11Nd18Fe4O39−δ and Nd18Li8Fe5O39 is discussed, and it is proposed that the sample prepared by Dutton et al. is a kinetic product whereas the sample prepared by Chen et al. is the thermodynamically preferred product.

A polycrystalline sample of La3Ni2SbO9 has been synthesized using a standard ceramic method and characterized by neutron diffraction and magnetometry. The compound adopts a monoclinic, perovskite-like structure with space group P21/n and unit cell parameters a = 5.0675(1), b = 5.6380(1), c = 7.9379(2) Å, β = 89.999(6)° at room temperature. The two crystallographically distinct six-coordinate sites are occupied by Ni2+ and a disordered distribution of Ni2+/Sb5+, respectively; the Ni2+ and Sb5+ cations occupy the disordered site in a 1:2 ratio. Both ac and dc magnetometry indicate the presence of a spontaneous magnetization below 105 K. A magnetization of 1.5 μB per formula unit was measured at 2 K in a field of 40 kOe. However, no magnetic scattering was observed in neutron diffraction data collected at 5 K. It is proposed that, as a consequence of the cation disorder, La3Ni2SbO9 behaves as a relaxor ferromagnet, analogous to a relaxor ferroelectric, with magnetic domains too small to be detected by neutron diffraction forming below 105 K.

A polycrystalline sample of Fe2GeMo3N has been synthesized by the reductive nitridation of a mixture of binary oxides in a flow of 10% dihydrogen in dinitrogen. The reaction product has been studied by magnetometry, neutron diffraction and Mössbauer spectroscopy over the temperature range 1.8 ≤ T/K ≤ 700. The electronic structure and magnetic coupling have been modelled by Density Functional Theory (DFT) and Monte Carlo methods. Fe2GeMo3N adopts the cubic η-carbide structure with a = 11.1630(1) Å at 300 K. The electrical resistivity was found to be ∼0.9 mΩ cm over the temperature range 80 ≤ T/K ≤ 300. On cooling below 455 K the compound undergoes a transition from a paramagnetic to an antiferromagnetic state. The magnetic unit cell contains an antiferromagnetic arrangement of eight ferromagnetic Fe4 tetrahedra; the ordered atomic magnetic moments, 1.90(4) μB per Fe atom at 1.8 K, align along a <111> direction. DFT predicts an ordered moment of 1.831 μB per Fe. A random phase approximation to the DFT parameterised Heisenberg model yields a Néel temperature of 549 K, whereas the value of 431 K is obtained in the classical limit for spin. Monte Carlo calculations confirm that the experimentally determined magnetic structure is the lowest-energy antiferromagnetic structure, but with a lower Néel temperature of 412 K. These results emphasise the potential of these computational methods in the search for new magnetic materials.

Nd18Li8Fe4TiO39 has been synthesised and characterised by neutron powder diffraction, X-ray absorption spectroscopy, Mössbauer spectroscopy and magnetometry. The cubic structure (Pm3̄n, a=11.97227(8) Å) is based on intersecting 〈1 1 1〉 chains comprised of alternating octahedral and trigonal-prismatic coordination sites. These chains lie within hexagonal-prismatic cavities formed by a Nd–O framework. The larger of the two crystallographically distinct octahedral sites, 8e, is occupied by iron, titanium and lithium in a ratio of 76:20:4; the smaller, 2a, is occupied by iron and titanium in a ratio of 79:21. The trigonal-prismatic site, 16i, is occupied by lithium and iron in a ratio of 98:2. The cations on the 2a sites are assigned as Ti4+and low-spin Fe4+, and those on the 16i sites as Li+ and Fe3+. The 8e sites are thought to be occupied by Li+, Fe3+ and Ti3+. Nd18Li8Fe4TiO39 undergoes a transition to a spin-glass state at 4.25(5) K.Graphical abstractNd18Li8Fe4TiO39 undergoes a transition to a spin-glass state at 4.25(5) K.Highlights► Spin-glass behaviour in Nd18Li8Fe4TiO39 below 4.25 K. ► Application of X-ray absorption spectroscopy to a complex mixed-metal oxide. ► Mixed-valence cations in Nd18Li8Fe4TiO39.

Nd18Li8Co3FeO39−y, Nd18Li8CoFe3O39−y and Nd18Li8Co3TiO39−y have been synthesised and characterised by neutron powder diffraction, magnetometry and Mössbauer spectroscopy. Their cubic structure (Pm3̄n, a∼11.9 Å) is based on intersecting <1 1 1> chains comprised of alternating octahedral and trigonal-prismatic coordination sites. These chains lie within hexagonal-prismatic cavities formed by a Nd–O framework. Each compound has an incomplete oxide sublattice (y∼1), with vacancies located around the octahedral sites that lie at the points of chain intersection. These sites are fully occupied by a disordered arrangement of transition-metal cations but only 75% of the remaining octahedral sites are occupied. The trigonal-prismatic sites are fully occupied by lithium except in the case of Nd18Li8CoFe3O39−y where some iron is present. Antiferromagnetic interactions are present on the Nd sublattice in each composition, but a spin glass forms below 5 K when a high concentration of spins is also present on the octahedral sites.Graphical AbstractCation and anion vacancies are found to coexist in mixed-metal oxides that adopt the La18Li8Rh5O39 structure.Highlights► Coexistence of anion and cation vacancies in compounds adopting the La18Li8Rh5O39 structure. ► Variation of the degree of magnetic frustration with the chemical composition in the La18Li8Rh5O39 structure. ► Controlling influence of the magnetic properties of the lanthanide cation becomes clear.

The chemical reduction of the K2NiF4-type oxides, Ln2Sr2CrNiO8−δ (Ln = La, Nd) and Nd2.25Sr1.75CrNiO8−δ, has been investigated in situ under a dynamic hydrogen atmosphere at high temperature using neutron powder diffraction. The high count-rate and high resolution of the D20 diffractometer at ILL, Grenoble allowed real-time data collection and structure refinement by full-pattern Rietveld analysis with a temperature resolution of 1 °C. Excellent agreement was obtained with the results of thermogravimetric analysis of these materials, which are potential fuel-cell electrodes. The neutron study revealed that oxygen is lost only from the equatorial anion site; the reduction of La2Sr2CrNiO8−δ yields a pure Ni(II) phase, La2Sr2CrNiO7.5en route to a mixed Ni(II,I) oxide, La2Sr2CrNiO7.40, whereas hydrogen reduction of Nd2Sr2CrNiO8−δ and Nd2.25Sr1.75CrNiO8−δ proceeds continuously from Ni(III) to an average oxidation state of 1.80 for the nickel ion. The data collected throughout a subsequent heating/cooling cycle in air indicated that the reduced phases intercalate oxygen reversibly into the equatorial vacancies of the K2NiF4-type structure. The retention of I4/mmm symmetry, along with the absence of the formation of any impurities throughout the heating/cooling cycles under reducing and oxidizing atmospheres, demonstrates both the reversibility and the strongly topotactic character of the oxygen deintercalation/intercalation chemical redox process and establishes the excellent structural stability of these layered mixed-metal oxides over a wide range of oxygen partial pressures.

Polycrystalline samples of Ln18Li8Rh5−xFexO39 (Ln = La, Nd; 0.5 ≤ x ≤ 5) have been synthesized by a solid-state method and studied by a combination of dc and ac magnetometry, neutron diffraction, and Mössbauer spectroscopy. All compositions adopt a cubic structure (space group Pm3̅n, a0 ∼ 12 Å) based on intersecting ⟨111⟩ chains made up of alternating octahedral and trigonal-prismatic coordination sites. These chains occupy channels within a Ln−O framework. At low values of x, iron preferentially occupies the smaller (2a) of the two distinct octahedral sites as low-spin Fe(IV). The Rh(III) on the larger (8e) octahedral site is replaced by high-spin Fe(III). Nd-containing compositions having x > 1 show spin-glass-like behavior below ∼5 K. La-containing compositions having x > 1 show evidence of a magnetic transition at ∼8 K, but the nature of the transition is unclear. This contrasting behavior demonstrates that, although the structural chemistry of the two systems is essentially the same, the magnetic character of the Ln cations plays an important role in determining the properties of these compounds.

Compositions in the series Ni2−xCoxGeMo3N (0 ≤ x ≤ 2), Co2Ge1−xGaxMo3N (0 < x ≤ 0.7), Co2−xFexGeMo3N (0 ≤ x ≤ 2), and Co2−xFexGe0.5Ga0.5Mo3N (0 ≤ x ≤ 0.8) have been synthesized by the reductive nitridation of binary oxides and studied by appropriate combinations of magnetometry, transport measurements, neutron diffraction, and Mössbauer spectroscopy. All of these compositions adopt the cubic η-carbide structure (a ∼ 11.11 Å) and show a resistivity of ∼10−3 Ω cm. No long-range magnetic order was observed in Ni2−xCoxGeMo3N, although evidence of spin freezing was observed in Co2GeMo3N. The introduction of gallium into this composition leads to the onset of antiferromagnetic ordering at 90 K in Co2Ge0.3Ga0.7Mo3N. The magnetic structure consists of an antiferromagnetic arrangement of ferromagnetic Co4 groups, with an ordered magnetic moment of 0.48(9) μB per cobalt atom. The same magnetic structure is found in Co0.5Fe1.5GeMo3N and Co1.2Fe0.8Ge0.5Ga0.5Mo3N. The former orders above room temperature with an average moment of 1.08(3) μB per transition-metal site, and the latter at 228 K with an average moment of 1.17(4) μB per site. The magnetic behavior of these compounds is discussed in terms of the electron count within each series.

Polycrystalline samples of La18Li8Rh4MO39 (M=Ti, Mn, Ru) have been prepared by a solid-state method and studied by neutron powder diffraction. They are isostructural with La18Li8Rh5O39 and adopt the cubic space group Pm3¯n with a ∼12.22 Å. Their structure consists of a La–O framework containing intersecting channels that run along 〈111〉. These channels are occupied by chains made up of alternating, face-sharing trigonal-prismatic and octahedral coordination polyhedra; there are two crystallographically distinct types of octahedral site. The prisms are occupied by Li and the transition metals are disordered over the two octahedral sites.The substitution of Ti, Mn or Ru into La18Li8Rh5O39 results in the loss of the cation ordering seen in the parent compound.

For the first time, the chemistry in H2 gas of a perovskite-like material, Pr2Sr2CrNiO8, has been monitored at temperatures up to ∼700 °C, in situ, by neutron powder diffraction.

A polycrystalline sample of Pr18Li8Fe4RuO39 has been synthesized by a solid state method and characterized by neutron powder diffraction, magnetometry and Mössbauer spectroscopy; samples of Pr18Li8Fe5−xMnxO39 and Pr18Li8Fe5−xCoxO39 (x  =1, 2) have been studied by magnetometry. All these compounds adopt a cubic structure (space group Pm3¯n, a0∼11.97 Å) based on intersecting 〈111〉 chains made up of alternating octahedral and trigonal-prismatic coordination sites. These chains occupy channels within a Pr–O framework. The trigonal-prismatic site in Pr18Li8Fe4RuO39 is occupied by Li+ and high-spin Fe3+. The remaining transition-metal cations occupy the two crystallographically-distinct octahedral sites in a disordered manner. All five compositions adopt a spin-glass-like state at 7 K (Pr18Li8Fe4RuO39) or below.Pr18Li8Fe5−xMxO39 (M=Ru, Mn, Co) have been studied by neutron diffraction, Mössbauer spectroscopy and magnetometry, allowing the distribution of the different cation species over the octahedral and trigonal-prismatic coordination sites within the structure to be determined. All the compositions studied undergo a transition to a spin-glass-like phase on cooling below ∼5 K.

Polycrystalline samples of Nd18Li8Fe5O39 and Nd18Li8Co4O39 have been synthesized using a solid-state method and studied by a combination of neutron diffraction, direct and alternating current magnetometry, and, in the case of Nd18Li8Fe5O39, Mössbauer spectroscopy. Both compounds adopt a cubic structure (space group Pm3̅n, a0 ∼ 11.9 Å) based on intersecting ⟨111⟩ chains made up of alternating octahedral and trigonal-prismatic coordination sites. The Fe4+ cations in Nd18Li8Fe5O39 are found on only the smaller of the two distinct octahedral sites in the structure; Fe3+ and Li+ are disordered over the larger octahedral site and the trigonal-prismatic site. The Nd3+ cations occupy sites between the chains. The smaller octahedral site is fully occupied by cobalt in Nd18Li8Co4O39, with 25% of the larger octahedral sites being vacant; Li+ is only found on the prismatic sites. Nd18Li8Fe5O39 shows spin-glass-like behavior with a spin-freezing temperature of 5.75 K, whereas Nd18Li8Co4O39 appears to order antiferromagnetically at 2.3 K. In both cases, the magnetic coupling involves the Nd3+ sublattice.

Polycrystalline samples of the n=1 Ruddlesden–Popper system Pr3−xSr1+xCrNiO8 have been synthesized over the composition range 0.00.1 adopt the tetragonal space group I4/mmm; Pr2.9Sr1.1CrNiO8 adopts the orthorhombic space group Fmmm. There is no evidence of Cr/Ni cation ordering in any composition. A maximum in the zero-field cooled magnetic susceptibility is observed at a temperature Tf that decreases with increasing Sr content; 52⩽Tf (K)⩽13. The frequency dependence of Tf observed in a.c. susceptibility measurements, together with the analysis of neutron diffraction data, suggests that the atomic magnetic moments in these compositions adopt a spin-glass-like state below Tf.The n=1 Ruddlesden–Popper system Pr3−xSr1+xCrNiO8 (0.0